As a wiring board on which semiconductor devices that are electronic component elements are mounted, a multilayer wiring board has been widely used in which wiring layers are formed on both surfaces of a core board. In recent years, with a reduction in the thickness of the multilayer wiring board and an increase in the density of wiring patterns, an increasing number of multilayer wiring boards with no core board have been provided. Advantageously, a multilayer wiring board with no core board (hereinafter also referred to as a “core-less board”) is easy to achieve thickness reduction and allows the wiring pattern to be densely formed.
As a method for manufacturing such a core-less board, a method for manufacturing a multilayer wiring board has been proposed which involves forming a laminate plate with desired numbers of insulating layers and wiring layers on each of the opposite surfaces of a support board that is a metal foil laminate plate, then separating the laminate plates from the support board, and executing subsequent steps on the separated laminate plates (see Patent Literatures 1 to 3). In this method, a multilayer wiring board is manufactured using a support board (hereinafter also referred to as a “dummy core”) used only for a manufacturing process and not included in the multilayer wiring board itself corresponding to a product. The method is hereinafter referred to as a core-less method.
However, the multilayer wiring board with no core board is inferior to a multilayer wiring board with a core board in the strength of the board itself and is thus easy to warp. Consequently, for example, Patent Literature 4 proposes that, in a multilayer wiring board comprising a central wiring layer disposed in the thickness-wise center of a board and a wiring layer laminated, via an insulating layer, on one side and the other side of the board across the central wiring layer, vias on the one and other sides of the central wiring layer each have a narrow cross-sectionally trapezoidal shape on a side of the via toward the central wiring layer. This multilayer wiring board is formed to be symmetric with respect to the central wiring layer to allow the insulating layers, wiring layers, and vias on the one and other sides across the central wiring layer to balance stress. This suppresses warpage.